I. Field of the Invention
The present invention relates to an aspheric spectacle lens blank having a flat top bifocal lens aligned with the vertical center line of the lens blank but spaced below the horizontal center line of the lens blank.
II. Description of the Prior Art
There are many types of previously known lens blanks particularly designed for aphakia patients, i.e. patients following extra capsular surgery. Many of these previously known lens blanks are substantially circular in shape and include an aspherical anterior surface with a gradient which decreases the power of the lens from the central area and toward the outer periphery of the lens. Prior to actual use by the patient, the outer periphery of the lens is trimmed to fit spectacle frames.
When these previously known lenses also include a bifocal segment, the bifocal segment is typically decentered from the geometric center of the lens blank, i.e., the bifocal portion is spaced both below the horizontal axis of the lens blank and is also horizontally spaced from the vertical centerline of the lens blank by a predetermined distance. This horizontal spacing from the vertical centerline of the lens blank is in one direction for a right lens and, conversely, in the other direction for a left lens. Consequently, after the lens blank is trimmed to fit the spectacle frames, the bifocal segments are positioned closer to the patient's nose in the well known fashion.
This previously known practice of manufacturing separate right and left lens blanks for lenses containing bifocal portions is disadvantageous in that it essentially doubles the amount of inventory of semi-finished lens blanks which must be maintained in inventory by the laboratory prior to trimming the blank to its final shape. In other words, for every left lens containing a bifocal segment, a right lens must also be maintained in inventory. Furthermore, in the event that one lens becomes scratched or flawed in any other fashion, the matching unflawed lens constitutes a wasted inventory cost, at least until the flawed lens can be replaced.